8 February 2015 Quantum cascade laser THz metrology
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The realization and control of radiation sources is the key for proper development of THz-based metrology. Quantum Cascade Lasers (QCLs) are crucial, towards this purpose, due to their compactness and flexibility and, even more important, to their narrow quantum-limited linewidth. We recently generated an air-propagating THz comb, referenced to an optical frequency comb by nonlinear optical rectification of a mode-locked femtosecond Ti:Sa laser and used it for phase-locking a 2.5 THz QCL. We have now demonstrated that this source can achieve a record low 10 parts per trillion absolute frequency stability (in tens of seconds), enabling high precision molecular spectroscopy. As a proof-ofprinciple, we measured the frequency of a rotational transition in a gas molecule (methanol) with an unprecedented precision (4 parts in one billion). A simple, though sensitive, direct absorption spectroscopy set-up could be used thanks to the mW-level power available from the QCL. The 10 kHz uncertainty level ranks this technique among the most precise ever developed in the THz range, challenging present theoretical molecular models. Hence, we expect that this new class of THz spectrometers opens new scenarios for metrological-grade molecular physics, including novel THzbased astronomy, high-precision trace-gas sensing, cold molecules physics, also helping to improve present theoretical models.
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P. De Natale, P. De Natale, L. Consolino, L. Consolino, D. Mazzotti, D. Mazzotti, A. Campa, A. Campa, M. Ravaro, M. Ravaro, M. S. Vitiello, M. S. Vitiello, S. Bartalini, S. Bartalini, "Quantum cascade laser THz metrology", Proc. SPIE 9370, Quantum Sensing and Nanophotonic Devices XII, 93701D (8 February 2015); doi: 10.1117/12.2084969; https://doi.org/10.1117/12.2084969

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